Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys. However, components formed from superalloys must be provided with some form of thermal and/or environmental protection in order to exhibit adequate service lives in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use. For superalloy components, oxidation-resistant aluminum-based intermetallic diffusion coatings such as platinum aluminide, and oxidation-resistant aluminum-containing overlay coatings such as MCrAlY (where M is iron, cobalt and/or nickel), are widely used as environmental coatings. These coating materials are also used to form bond coats to adhere a TBC, which is typically a metal oxide such as zirconia (ZrO.sub.2) that is partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides.
While superalloys have found wide use for components throughout gas turbine engines, alternative materials have been proposed. Materials containing silicon, particularly those with silicon carbide (SiC) as a matrix material or a reinforcing material, are currently being considered for high temperature applications, such as combustor and other hot section components of gas turbine engines. In many applications, a protective coating over the Si-containing material is beneficial. For example, protection with a suitable thermal-insulating layer reduces the operating temperature and thermal gradient through the material. Additionally, such coatings can provide environmental protection by inhibiting the major mechanism for degradation of silicon carbide in a corrosive environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH).sub.4) products. On this basis, besides low thermal conductivity, a critical requirement of a thermal barrier coating system for a SiC-containing material is low activity of silica (SiO.sub.2) in its composition. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, and chemical compatibility with SiC and silica scale. Consequently, the coating essentially has a dual function, serving as a thermal barrier and simultaneously providing protection from the environment. A coating system having this dual function may be termed a thermal/environmental barrier coating (TBC/EBC) system.
While various coating systems have been investigated, each has exhibited shortcomings relating to the above-noted requirements and properties for compatibility with a Si-containing material. For example, an yttria-stabilized zirconia (YSZ) coating serving as a thermal barrier layer exhibits excellent environmental resistance by itself, since it does not contain silica in its composition. However, YSZ exhibits high permeability to oxygen and other oxidants. In addition, YSZ cannot be adhered directly to silicon carbide because of a CTE mismatch. As a result, mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) has been proposed as a bond coat between a SiC-containing substrate material and a ceramic TBC in order to compensate for differences in CTE. However, mullite exhibits significant silica activity and volatilization at high-temperature exposures to a water vapor-containing environment. This can especially be the case if the YSZ TBC is deposited by electron beam physical vapor deposition (EBPVD) techniques, and consequently has a columnar grain structure that is permeable to oxidant species. Accordingly, there is a need for an improved TBC/EBC system for Si-based materials.